Do auroras count as an "optical" phenomenon?

Solution 1:

Here are relative prevalences in Google Books for some different possible associations...

aurora "electromagnetic phenomenon" 96 hits
aurora "visual phenomenon" 99 hits
aurora "solar phenomenon" 320 hits
aurora "optical phenomenon" 588 hits
aurora "atmospheric phenomenon" 1100 hits

I think what that tells us is that regardless of any distinction OP may draw between visual and optical, most writers prefer the latter when referencing aurora.

Personally I don't recognise OP's distinction as meaningful anyway. Obviously optical illusions don't usually involve "the behavior of light through different media", but as this GoogleFight shows, they wipe the floor with visual illusions.